The invention relates to a power supply unit for generating a protective current (sealing current), which is superposed on signal currents carried by metal lines for protection against corrosion. The invention also relates to a process for generating such a protective
The metal lines used for transmitting signal currents, e.g., the copper lines in telephone networks still commonly in use for ISDN terminals, are exposed to corrosion at splices or tie points, which over time results in a deterioration in conductivity along the employed line.
However, it was found that feeding in a low direct current can prevent corrosion of the lines.
Primarily for this reason, a protective current (sealing/wetting current) of several milliamperes is superposed on the irregular and direct current-free data stream via the copper lines in digital subscriber terminal systems that use metal line loops as in the ISDN base terminal (BA).
In systems where a remote feed is set upon the user to supply power to the network terminating unit (NT), the direct current can perform the same function as the protective current on the line resulting from the power consumption by the network terminating unit. By contrast, an application provide for protective current supply is normally not used to supply power to network terminating units.
Another potential application for protective current is to use it as a line detection signal for network terminating units (NT). In an ISDN network, the network terminating unit on the user side forms the transition between the basic terminal interface (U interface), which establishes the connection to a central office, and the standardized S0 interface, to which users can hook up their terminals.
According to standard ANSI T1.601-1992, which describes the basic terminal interface when using metal line loops in ISDN applications, it is not absolutely necessary to provide a protective current. However, if a protective current is provided, certain requirements have to be met. If it does not measure 0 mA, the protective current must exhibit values ranging between 1 mA and 20 mA. In addition, the maximal allowable change rate for the protective current is 20 mA/s.
Other requirements are placed on the metal terminating unit (metallic termination) of a network terminal device, which provides a non-linear direct current path as the sink for the protective current fed in via the terminal lines. The operating states of the metal terminating unit are the non-conducting state OFF (high impedance) and conductive state ON (low impedance).
If a voltage to range between 30 V and 39 V is applied for a preset time to range between 3 ms and 50 ms in the OFF state of the metal terminating unit, a switch is made to the ON state. In this case, the state is to switch within at most 50 ms starting from the point the preset voltage was exceeded. A state change does not take place even at voltages exceeding the preset voltage if it is exceeded for less than 3 ms.
If the current drops below a preset current value (0.1-1.0 mA) for a preset time (3-100 ms) in the ON state of the metal terminating unit, a switch is to be made to the OFF state. In this case, the state is to change within at most 100 ms starting from the point at which levels dropped below the preset current. A state change does not take place even at currents less than the preset current is it levels remain below the preset current for less than 3 ms.
The process of feeding a protective current into the data transmission lines must not disrupt ongoing transmissions. For this reason, it is essential that a stable ON state be ensured for the metal terminating unit of a network terminal unit, and that current changes not arise too abruptly.
Therefore, the power supply unit in, the line terminal unit must satisfy several special requirements to reliably ensure a stable operation of the metal terminating unit of the network terminal unit on the user side.
The object of the invention is to provide a power supply unit that is particularly suited for delivering ANSI T1.601-1992-conformant protective currents (sealing currents) for digital user terminal lines.
The object is solved according to the invention on the one hand by a power supply unit for generating a protective current (sealing current), which is superposed on the metal lines via signal currents transmitted via metal lines for protection against corrosion at tie points or splices; this power supply unit
exhibits two voltage-controlled power sources, wherein the first power source is capable, depending on an applied voltage, of outputting a constant current having a first value of 0 mA, or a constant current having a second value of at least 1 mA, in particular 1.2 mA, and wherein the second power source is capable, under a threshold value, in particular of 0 V, of the applied voltage, of outputting a constant current having a third value of 0 mA, and, above the threshold value of the applied voltage and proportional to the latter, a current between the constant current of the third value and a fourth value of 20 mA diminished at least by the constant current having the second value of the first power source,
exhibits at least one voltage input, which is connected with the control input of the second power source via an integrator, and
exhibits a current output, with which both the output of the first and the output of the second power source are connected for outputting the sum of the currents delivered by both power sources.
In addition, the object is achieved according to the invention by a process for generating a protective current (sealing current) for protection against corrosion of splices and/or tie points of metal lines via a power supply unit, which comprises a first and second voltage-controlled power source, wherein the first power source is capable, depending on an applied binary control voltage, to generate a constant current having a first value of 0 mA, or a constant current having a second value of at least 1 mA, in particular 1.2 mA, and wherein the second power source is capable, at below a threshold value, in particular of 0 V, to generate a constant current having a third value of 0 mA, and at above the threshold value of the applied voltage and proportional to the latter, a current between the constant current having the third value and a fourth value 20 mA diminished at least by the constant current having the second value of the first power source, characterized by the following steps:
application of an input voltage to the voltage input of the power supply unit,
generation of a binary input voltage from the applied input voltage, unless binary application of the latter has already taken place,
integration of the binary input voltage, in particular via RC integration,
shifting of integrated voltage by a negative offset voltage,
generation of binary control voltage by comparing the input voltage, integrated input voltage or integrated and shifted input voltage with a reference value using a comparator,
generation of an initial output current via the first power source, controlled by the binary control voltage,
generation of a second output current via the second power source, controlled by the integrated and shifted voltage,
addition of the first and second output current into a total current to be used as a protective current, and
output of the protective current on the metal lines to be protected against corrosion.
The binary control voltage should assume a high level during acceleration, before the shifted voltage exceeds the threshold value, and assume a low level during deceleration, before the shifted voltage again drops below the threshold value.
With the power supply unit and process according to the invention, it is possible to provide a protective current that satisfies the ANSI T1.601-1992 requirements.
The power supply unit according to the invention can be operated in such a way as to generate and output a protective current that measures either 0 mA, or runs between a valuexe2x89xa71 mA, e.g., 1.2 mA, and a valuexe2x89xa620 mA.
The protective current generated and output using the process according to the invention starts at a valuexe2x89xa71 mA, e.g., 1.2 mA, when applying a voltage with a high level to the voltage input of the power supply unit, rises to xe2x89xa620 mA and drops, and then drops back down to a valuexe2x89xa71 mA after applying a voltage with a low level to the voltage input of the power supply unit. An abrupt transition from 0 mA to the valuexe2x89xa71 mA can be achieved at the outset, followed at the end by a direct transition from the valuexe2x89xa71 mA to 0 mA. This kind of current progression can also be achieved with the power supply unit according to the invention. The maximal change rate of the current generated by the second power source, and hence also of the protective current, can be set in such a way by appropriately dimensioning the integrator and at the offset voltage level that a change rate of 20 mA/s is never exceeded with the power supply unit active.
In addition, the power supply unit and process according to the invention can be used to satisfy the ANSI T1.601-1992 requirements relating to the metal terminating unit of a network terminal unit.
Immediately after the metal terminating unit of the network terminal unit has gone from is OFF to its ON state, the power supply unit can deliver a direct current that exceeds the release current set at a value of between 0.1 mA and 1.0 mA (ANSI T1.601-1992), below which the metal terminating unit can again switch to the OFF state. Once the metal terminating unit has been turned on, the rate of rise for the protective current, which rises to a maximum of 20 mA, is limited to a maximum of 20 mA/s given appropriately set parameters. Due to the exponential rise of the current achieved via the provided integration, the maximum of 20 mA can also be reached and maintained without any irregular transitions.
In addition, the rate of decline for the protective current as it drops from its maximal value of 20 mA to a value slightly exceeding the set minimal value of the protective current of 1 mA, below which the metal terminating unit again switches to the OFF state, can be limited to a maximal 20 mA/s. Due to the exponential drop in current, the minimal value of the protective current can also be reached and maintained without any irregular transitions.
The transition of the metal terminating unit from its ON state to its OFF state is reached starting front the final value of the protective currentxe2x89xa71 mA by virtue of the fact that the supplied protective current drops off in a strictly monotonic manner.
If the metal terminating unit in the OFF state is to be activated, the no-load voltage of the power supply unit is set in such a way that it exceeds the set upper limit for the activation voltage of the metal terminating unit of 39 V.
By contrast, after switching from the ON to the OFF state of the metal terminating unit, once the shortest possible release time of 3 ms has elapsed, the no-load voltage of the power supply unit, which now delivers no more current, is held under the lowest activation voltage of 30 V, to prevent an immediate reactivation of the metal terminating unit. By deactivating the power sources, this can be automatically ensured.
In a preferred embodiment of the power supply unit according to the invention, a level shifter is arranged between the integrator and control input of the second power source, wherein the level shifter is capable of shifting the voltage by a negative offset voltage. An analog arrangement of other components can also be provided instead of a level shifter. If the voltage output by the integrator is applied to the input of the comparator, the magnitude of the offset voltage of the level shifter preferably exceeds the reference voltage of the comparator, so that the first power source is activated already a short time before the second power source.
One particularly flexible potential application for the power supply unit and process according to the invention is achieved with a first power source that can be activated on the one hand by the binary voltage delivered by the comparator, and on the other hand by a voltage that can be separately applied and made available in binary form.
This provides the operator with three different types of operation:
In a first type of operation, the voltage can be permanently set to a low level at the input connected with the integrator, and the voltage at the other voltage input can be permanently set to a high level, as a result of which a constant, low direct current greater than 1 mA, in particular 1.2 mA, is output.
This type of operation is primarily of importance for low-power applications, in which a high protective current cannot be used, but it must still be possible to stably operate the metal terminating unit of an ANSI T1.601-1992 conformant network terminal unit in the ON state, while the low protective current is applied to the line.
In a second type of operation, the voltage at the first voltage input can be continuously set to a high level, and the voltage a the additional voltage input can be continuously set to a low level. In this case, the standard output current of the power supply unit is 20 mA. The power supply unit is deactivated only for maintenance operations, by also setting the voltage at the first voltage input to a low level. This is followed by an exponential drop to the valuexe2x89xa71 mA, and after a short time, a step-by-step transition to 0 mA. Once the power supply unit is reactivated by again continuously setting the voltage at the first voltage input to a high level, a jump first takes place to a valuexe2x89xa71 mA, e.g., 1.2 mA, followed a short time later by an exponential rise to 20 mA.
Finally, a third type of operation can involve a mixed operation, in which the power supply unit basically outputs a low current with a valuexe2x89xa71 mA, by permanently applying a high level to the first power source via the additional voltage input. A voltage with a low level is again applied to the integrator under normal conditions. This time, however, this voltage applied to the integrator is set to a high level at regular intervals and for a specific duration. This results in an exponential rise to xe2x89xa620 mA after a short delay, depending on the value of the offset voltage of the level shifter. After the voltage applied to the integrator has again assumed a low level, the output current drops exponentially to the valuexe2x89xa71 mA. Such a mixed operation is preferred worldwide by several network operators. The protective current rise is usually repeated in intervals of about 15 minutes for a duration of about 20 seconds.
In an advantageous embodiment of the power supply unit according to the invention, a binary voltage is already applied to the voltage input and/or to the additional voltage input of the power supply unit. As an alternative, however, the applied voltages can also be converted into binary voltages only inside the power supply unit, e.g., via a Schmitt trigger. The binary voltages applied to the inputs or generated internally can be amplified and/or shifted via additional level shifters downstream from the voltage inputs. Particularly suited high and low levels of the binary voltage signals for continued processing, e.g., integration and OR operation, can be set through such a transformation to another binary voltage level, independently of the input voltages.
Corresponding control software is advantageously used to control the voltages to be applied to the inputs of the power supply unit, e.g., integrated in the microprocessor of the user terminal card. Software control is particularly easy to implement. However, it can also take place in any other manner desired, e.g., via hard-wired logic.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood. however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.